1. Field of the Invention
This invention relates generally to the field of magnetron sputtering devices, and more specifically to circular planar magnetron sputtering devices which have improved magnetic field structures and control features for the vacuum deposition of thin films of magnetic and non-magnetic materials on substrates. Products benefiting from application of this technology include but are not limited to computer memory disks, recording heads, and integrated circuits.
2. Brief Description of the Prior Art
Magnetron sputtering is a well-known vacuum coating process which utilizes an electrical glow discharge (or plasma) confined by magnetic fields to erode a material from a target and deposit that material onto the surface of a substrate. Its use in the electronics and computer industries is wide spread and includes such products as memory disks, audio tape, recording heads, integrated circuits, and photovoltaic cells to name but a few. Large-scale devices are used to deposit a variety of coatings on architectural glass, computer screens, automobile windows, and other substrates.
The earliest plasma sputtering apparatus took the form of a diode. Sputtering is done in a vacuum chamber in the presence of a small amount of an inert sputtering gas (usually argon). The planar target material to be sputtered is connected to the negative electrode of a DC power supply while the positive electrode is connected to a separate nearby anode or to the chamber itself, which can serve as an anode. The electric field developed between the target material and the anode is typically several thousand volts, and is sufficient to cause a fraction of the argon sputtering gas to be ionized to form a plasma discharge. The positive argon ions bombard the negative target dislodging additional electrons and target material atoms, which deposit on a substrate placed a small distance away. Deposition rates are low and the process is inefficient because, at the relatively high argon gas pressure needed to form the plasma discharge, the mean free path of the ions is only a small fraction of a centimeter. This means that the argon ions must undergo many collisions with other argon atoms on their way to the target, losing energy that would otherwise go into dislodging target atoms.
The efficiency of the diode sputtering process was greatly improved by the addition of a magnetic assembly placed behind the target material. The magnets are arranged to form a closed loop, magnetic tunnel on the target material surface. The magnetic tunnel efficiently traps electrons and holds them close to the target surface. The resulting improvement in ionization efficiency of the argon gas allows the pressure to be reduced thus increasing the mean free path of the argon ions in the plasma. The increased mean free path results in less energy loss from collisions as the argon ions are accelerated into the target surface by the negative voltage potential. The sputtering of the target material is increased in the region of the magnetic tunnel, and the resulting erosion zone is universally referred to as the "racetrack".
The most commonly used magnetron sputtering device is the planar magnetron. The term planar describes the target material, which, in the vast majority of cases, takes the form of a rectangular or circular flat plate. The invention of the device is credited to Chapin in U.S. Pat. No. 4,166,018 and entitled "Sputtering Process and Apparatus". Magnets positioned behind the target plate produce a magnetic tunnel, which confines the plasma and defines the sputtering racetrack. Several types of magnetic assemblies exist, which shape the plasma into a closed loop whose cross-sectional shape depends upon the particular details of the magnetic design. Additionally, magnetic assemblies designed for the sputtering of non-magnetic materials usually are less complex than those designed for the sputtering of magnetic materials.
Although magnetron sputtering has become a key technology used in many commercial applications, problems remain to be solved and improvements continue to be made. The sputtering process is inherently very inefficient. On average over 80% of the energy of each accelerated argon ion that strikes the target surface goes into heating the target, and less than 20% to ejecting target atoms. Therefore, all magnetrons must have an efficient target cooling system to stabilize the process at a relatively low temperature. In addition, most simple magnetic assemblies produce racetrack erosion zones that are sharp and narrow, leading to low utilization of the target material.
In general a cross-sectional view of the construction details of a circular planar magnetron will appear identical to that of a rectangular planar magnetron. Indeed, a rectangular planar magnetron may be visualized as a circular magnetron that has been cut in half with a linear extension of the same cross-sectional construction placed between the two halves. However, a design that is advantageous for one type of device may not be as useful for the other. The reason for this is that the two types of magnetrons are usually operated differently when coating substrates. The rectangular planar magnetrons are most often employed for coating large, rectangularly shaped substrates in a dynamic or "pass-by" mode. The round ends of the magnetron are generally shielded, or at least extend past the substrate at each end, to improve coating uniformity perpendicular to the direction of motion of the substrate. A non-uniform deposition pattern across the width of the magnetron is of little or no consequence on coating thickness uniformity because of the averaging effect of the substrate motion. However, a non-uniformity in chemical composition as a function of coating thickness still exists for coatings formed by reactive sputtering. For example, this subtle, but important effect can cause degradation in the quality of transparent, electrically conducting coatings.
Circular planar magnetrons are used to coat round, wafer-like substrates placed symmetrically in front of the target, and held stationary with respect to the target during the deposition. This is commonly referred to in the industry as "static" mode. Since there is no substrate motion to help average out non-uniformities, much more care in the details of the magnetron design is required if uniform coatings are to be obtained. Simply rotating the substrate about its center does not improve the uniformity, because the deposition pattern is radially symmetric. A circular magnetron, which has very uniform erosion of the target material across its surface, will not coat a round substrate uniformly unless the diameter of the magnetron is much greater than the diameter of the substrate, or unless the substrate is placed very close to the target surface. Neither of these solutions is practical. In the first instance, the targets must be so much larger than the substrate that the net efficiency of utilization of target material is very poor. In the second instance, the substrate is so close to the target that substrate heating is too severe. The problems with the circular planar magnetron are therefore uniquely different from those of the rectangular planar magnetron. The challenge for the circular planar magnetron is to find a design that gives uniform coatings throughout target life, while obtaining reasonable target utilization in a format where the diameters of the magnetron and substrate are substantially the same, i.e. less than about a factor of two different. For example, in the magnetic memory disk coating industry, 95 mm diameter disks are coated with sputtering targets that are about 150 mm in diameter.
Some of the more recent improvements in magnetron designs include U.S. Pat. No. 4,865,708 issued to Welty and entitled "Magnetron Sputtering Cathode". This patent describes a method for improving target material utilization that changes the usual convex shape of the magnetic field lines, which form the magnetic tunnel, to a slightly concave shape, which results in a broader sputtering groove. The methods used include locating magnetically permeable material between the poles of the usual permanent magnets, as well as combinations of permanent magnets, electromagnetic coils, and permeable material arranged to accomplish a similar result. The prior art for the preferred embodiment is shown in FIG. 1 as a cross-sectional view. Permanent magnets 1 are placed on magnetically permeable pole piece 2 to form magnetic field structure 3. Magnetically permeable elements 4 cause magnetic field lines 5 to become slightly concave in the region of target 6, thus broadening the sputtering groove. No method is provided to allow adjustment of the magnetic field profile during the life of the target, and there is no data on coating uniformity.
The improvement by Manley in U.S. Pat. No. 4,415,754 entitled "Method and Apparatus for Sputtering Magnetic Target Materials" is shown in the cross-sectional view of FIG. 2. Elements common to those in FIG. 1 are labeled with the same numerals. Although FIG. 2 is similar in appearance to FIG. 1, the function of magnetically permeable material 2 (called a magnetic shunt) is different because target material 6 is magnetic. As target 6 erodes, the strength of magnetic field lines 3 would normally intensify rapidly. This would cause the sputtering groove to become increasing sharp and narrow, yielding very poor target material utilization. The addition of magnetic shunt 4 shorts some of the field as the target erodes and improves target utilization. However this design, like the others, does not provide for any magnetic field adjustment during operation. Data for coating uniformity on statically sputtered substrates is not discussed.
Another way to improve the utilization of the target material is to move the magnetic assembly (which moves the sputtering groove) during operation of the device. An example of this method is shown in FIG. 3 as a plan view. This device is described by Demaray et al in U.S. Pat. No. 5,252,194 entitled "Rotating Sputtering Apparatus for Selected Erosion". Closed loop magnetic assembly 7 is positioned asymmetrically with respect to circular target position 6a. During operation magnetic assembly 7 is made to rotate around the center of target position 6a, thus sweeping the sputtering groove formed by the magnetic assembly over substantially the entire surface of the target. While target utilization is improved by this design, problems still exist with deposition uniformity for static substrates. Additionally, if the target material is magnetic, eddy current losses increase as the speed of rotation of the magnetic assembly is increased. This can be a problem for short deposition times.
Still another improvement to the planar magnetron was described by Manley in U.S. Pat. No. 5,262,028 entitled "Planar Magnetron Sputtering Magnetic Assembly". A cross-sectional view illustrating the method is shown in FIG. 4. Elements common to those of previous figures are labeled with the same numerals. The device uses conventional magnets 1 oriented with their poles in opposite (normal) directions, and perpendicular to the plane of pole piece 2. However, in this case, pole piece 2 is not formed in a simple flat geometry but it is shaped in a particular way to accommodate an auxiliary array of magnets la oriented with their poles parallel to the plane of pole piece 2. The magnets and the shape of the pole piece cause field lines 3 in the region of the sputtering groove of target 6 to flatten and become concave. As in other methods previously discussed, this causes the sputtering groove to broaden and target utilization to be improved. Adjustment of the magnetic field during operation is not allowed for in the design, and coating uniformity is not discussed.
A design for a magnetic assembly which uses a series of concentric electromagnetic coils is described by Potter in U.S. Pat. No. 5,262,030 entitled "Magnetron Sputtering Cathode with Electrically Variable Source Size and Location for Coating Multiple Substrates".
The device is further described in a paper by Potter and Wilson entitled "Performance of a Variable-plasma Computer-controlled Sputtering Source", Surface and Coatings Technology, 72(1995) 196-199. A cross-sectional view of this device is shown in FIG. 5. Pole piece 2 is relatively thick in comparison to the preceding devices, and it contains a concentric series of gaps 8, which support a series of independent electromagnetic coils 9. Target material 6 is held in close proximity to the yoke portions of pole piece 2, which are formed by the material left between gaps 8. Permanent magnets are not used in the device. By energizing various sets of coils, at relatively high current levels, the magnetic field that forms the sputtering groove may be swept radially across the face of the target to spread the sputtering groove and increase target utilization. Alternative types of sweep patterns also are discussed in the patent.
While the magnetron works in principle and has a broad range of adjustability, it is both mechanically and electrically complex to construct and operate.
What is needed is a simplified design for a circular planar magnetron, which provides reasonably good target utilization, and at the same time allows for control of sputtering groove positions and erosion rates during operation. This is necessary to maintain thickness uniformity in statistically deposited coatings throughout the lifetime of the target.
All of the patents cited above are hereby incorporated by reference for purposes of additional disclosure.